1. Field of the Invention
The present invention relates to an onium salt containing as a photo acid generator a specific thioxane skeleton at a cation moiety; a resist material for exposure to high energy rays having a wavelength preferably of 300 nm or less, containing this onium salt; and a pattern formation method using this resist material.
2. Description of the Related Art
Recently, with a higher degree of integration and higher speed of LSI, fine structure of a pattern rule is desired, and under such conditions, far ultraviolet lithography and vacuum ultraviolet lithography are regarded as promising as fine processing technologies of the next generation.
Currently, tip semiconductors of a 0.15 μm rule are produced by photolithography using a KrF excimer laser, and a 0.13 μm rule is also at the initial stages of production. Realization of photolithography using an ArF excimer laser ray as a light source is eagerly desired as a technology essential to ultra-fine processing of 0.13 μm or less.
Particularly, in photolithography using an ArF excimer laser ray as a light source, resist materials having high sensitivity capable of manifesting sufficient resolution with a small exposure amount, for preventing deterioration of precise and expensive optical materials has been demanded. As a device for realizing resist materials of high sensitivity, it is most common to select that which is highly transparent at a wavelength of 193 nm as each composition. For example, with respect to base resins, there are suggested poly(meth)acrylic acid and derivatives thereof, a norbornene-maleic anhydride alternating polymer, polynorbornene and metathesis ring-opened polymer, and the like, and effects of certain extent have been obtained from the standpoint of an increase in transparency of a resin single body. However, regarding the acid generator, when transparency increases, acid generation efficiency decreases, resulting in low sensitivity, or leading to deficiency in heat stability and storage stability, that is, those satisfying practical requirements are not obtained yet at present. (Meth)acrylic acid is an abbreviation for methacrylic acid and/or acrylic acid, therefore, poly(meth)acrylic acid means polymethacrylic acid and/or polyacrylic acid.
For example, alkylsulfonium salts suggested in Japanese Patent Provisional Publication Nos. 7-25846/1995 (U.S. Pat. Nos. 5,585,507 and 5,635,332), 7-28237/1995 (U.S. Pat. Nos. 5,585,507 and 5,635,332), 8-27102/1996 and 2000-292917, and the like have very high transparency, while, having insufficient acid generation efficiency and having difficulty also in heat stability. Thus, they are not satisfactory. Alkylarylsulfonium salts suggested in Japanese Patent Provisional Publication No. 10-319581/1998, and the like have good balance between transparency and acid generation efficiency and have high sensitivity, although deficient in heat stability and storage stability. The arylsulfonium salts which have been effective in photolithography using a KrF excimer laser ray are excellent in acid generation efficiency, heat stability and storage stability. However, they are remarkably low in transparency, and patterns after development are in the form of significant taper. Although there is a device for decreasing film thickness of a resist for compensating its transparency, this case leads to a remarkable decrease in the etching resistance of a resist film, namely, this device is not suitable as a pattern formation method. These are cases in which mainly the structure of the cation side of an onium salt is changed, and there is a report that the kind of acid generated and the kind of acid-labile group are in a tight relation, in resolution and pattern formation.
With promotion of fine structure, a difference in dimension among line-edge roughness, isolated pattern and dense pattern (I/G bias) is becoming problematic. It is conventionally well-known that even if dimensions on a mask are equivalent, a difference in dimensions occurs between the dense pattern and isolated pattern after development. In dimensions over the wavelength, the above-described problem is serious.
The reason for this is that optical strength varies depending on a difference in light interference in image formation of the dense pattern and isolated pattern. For example, FIG. 1 shows dimensions when the pitch of 0.18μ line is changed under optical conditions of a wavelength of 248 nm, NA of 0.6 and σ of 0.75. When standardized so that the line dimension is 0.18 μm at a pitch of 0.36 μm (0.18 μm line, 0.18 μm space), the dimensions of optical images once narrows and then broadens with expansion of pitch.
Next, results measuring the resist dimension after development are also shown. For the resist dimension, simulation software PROLITH 2 Ver. 6.0 available from KLA-Tencor Corporation (formerly, Finle Technologies Inc.) was used. The resist dimension narrows with expansion of pitch, further, narrows increasingly with an increase in acid diffusion. A problem of diffuseness/denseness dependency in which the dimensions of an isolated pattern narrow as compared with those of a dense pattern is becoming of great concern. It is understood from the above-described simulation results that a method of decreasing acid diffusion is effective as the method of decreasing diffuseness/denseness dependency. However, when the acid diffusion is too small, there occurs a problem that the side wall of a resist pattern after development becomes uneven and rough skin caused by standing wave, or line-edge roughness increases. For example, FIG. 2 shows the results of calculation of the cross-sectional form of a resist of an 0.18 μm line and space pattern on the Si substrate when the acid diffusion distance is varied using the above-described simulation software PROLITH Ver. 6.0 available from KLA-Tencor Corporation. It is shown that when the acid diffusion distance is smaller, unevenness of the side wall caused by standing wave is remarkable. Also regarding line-edge roughness observed from above SEM, the same tendency is shown, namely, when acid diffusion is smaller, line-edge roughness increases further. For decreasing the roughness of a line, it is common to increase acid diffusion distance. However, diffuseness/denseness dependency cannot be improved further by this. As the method of improving line-edge roughness, methods of improving light contrast are mentioned. For example, at the same exposure wavelength, when the dimension of line width is larger, line-edge roughness decreases. Even at the same exposure wavelength and the same dimension, when NA of a stepper is higher, smaller line-edge roughness is obtained in off-axis illumination (for example, annular illumination and quadrupole illumination) than in normal illumination, usually, on a phase shift mask than on a Cr mask. The contrast and line-edge roughness of line-edge of a pattern are correlated, and when line-edge contrast is steeper, line-edge roughness is smaller. Further, it is supposed that smaller line-edge roughness is obtained in the case of exposure to shorter wavelength. When line-edge roughness in KrF exposure and line-edge roughness in ArF exposure are compared, optical contrast is expected to be higher by shorter wavelength in ArF exposure, and line-edge roughness to be small. However, there is a report that KrF exposure is actually far superior (SPIE 3999, 264, (2000)). This is based on a difference in abilities of KrF resist materials and ArF resist materials, and indicates that, in particular, line-edge roughness derived from materials in the ArF exposure is of concern, and it is desired to obtain an acid generator that does not worsen diffuseness/denseness dependency while improving line-edge roughness, simultaneously.
Though 2-oxoethylarylthiacyclopentanium salt shown in the above-described Japanese Patent Provisional Publication No. 2000-292917 is suggested as that which is excellent in sensitivity and still further excellent in rectangularity of a resist pattern, it has been found that it is not satisfactory as an acid generator that does not worsen diffuseness/denseness dependency while improving line-edge roughness, simultaneously.